It is known that, in the preparation of suspension polyvinyl chloride for use in hard PVC compositions, working adjuvants such as stabilizers, fillers, pigments and polymeric impact strength improving agents can be added to the batch prior to polymerization. These are the same additives which are put into pure PVC in conventional methods of fabrication. If all of the additives which are necessary for fabrication can be added to the polymer right in the autoclave, any separate mixing process prior to fabrication can be eliminated.
The size and shape of the PVC composition particles are important to the manufacturer. The product should be free-flowing. Large amounts of particles smaller than 50.mu. militate against free flow and are therefore undesirable. Furthermore, thread-like and scale-like components must be avoided because in many cases they result in blemishes in extruded products and sheets. If the product to be taken from the autoclave is to be a PVC composition that is ready for fabrication, a number of substances are added to the polymerization mixture according to the future application of the composition. It is obvious that in this method the danger of the formation of undesired particle shapes and sizes is particularly great, since the boundary surface forces of the VC-water system, which is determinative of the later particle shape, are more or less greatly affected by the substances added.
In the case of fines in the polymer, there is also the danger that they may be additive substances which were suspended in the aqueous phase. These fines can separate in pneumatic conveying processes and thus cause inhomogeneities. For this reason, too, a very uniform grain size and composition in the individual grains is of the utmost importance.
The previously known method of polymerizing suspension PVC compositions suffers from decided deficiencies and therefore is little used or is restricted to special, simple products.